Systems for protecting an optical electromagnetic wave-responsive device, such as an optical sensor, from a high intensity laser beam are well known as shown, for example, in U.S. Pat. No. 4,829,269 to Minahan. There, radiation is transmitted to the sensor through a nonlinear crystal and plasma switch. A harmonic component is produced at the nonlinear crystal, which component increases with increasing intensity of the incident radiation. The harmonic component from the nonlinear crystal initiates a state of nontransmittance of the plasma to block transmission to the sensor. The plasma is switched from a state of transmittance to a state of nontransmittance at an intensity level of the incident radiation below which damage to the sensor would occur.
In U.S. Pat. No. 4,431,257 to Born a device for protecting an optical or infrared window against damage by laser radiation is shown which includes a coolant flow channel for the flow of a gaseous or liquid coolant which is opaque to the laser radiation and transparent to visible and near-infrared radiation. The coolant comprises either C.sub.6 H.sub.12 or CH.sub.4.
A diffusion filter for a laser beam is shown in U.S. Pat. No. 3,587,424 to Paine, which filter comprises a laser light transparent receptacle containing a suspension of small particles having an average diameter larger than the wavelength of the laser light. A colloidal suspension of milk or metal particles in water is employed in the receptacle, which particles reflect laser light for diffusion of the laser light beam.
A substantially stable colloidal suspension of carbon black particles of an average size under 20 .mu.m for use in rear view mirrors is shown in U.S. Pat. No. 3,509,063 to Teague et al. The suspension liquid comprises a solution of an organic liquid, together with a dispersing agent. The suspension is located in an enclosure with a window and containing a movable mirror. By moving the mirror away from the window the layer of suspension between the mirror and window is increased to produce an opaque screen in front of the mirror to inhibit the reflection of light from the mirror.
An optical limiter using Cr-doped crystalline solids and organic solutions, such as laser dyes, saturable absorber dyes, etc. is shown in Harter et al, U.S. Pat. No. 4,723,248.
In Seitel et al, U.S. Pat. No. 4,597,639, an optical radiation attenuating device is shown comprising either a mirror or optical transmitting component upon which the radiation impinges. Disposed on the surface of the device are particles of material which emit electrons when the power of radiation impinging thereon is above an optical breakdown threshold. Above the threshold level, free electrons emitted from the particles ionize the adjacent gas, or atmosphere, to provide a shielding plasma. A nanosecond response time is provided by the plasma shield.
An optical radiation limiter is shown in U.S. Pat. No. 4,890,075 to Pohlmann et al, which includes a fluid, such as methanol, within which carbon particles are suspended. Radiation to be limited is focused at the suspension, and transmitted radiation is limited to a level below the detector damage level. Carbon particles absorbing high-intensity energy either form radiation scattering vapor bubbles around themselves, and/or become hot plasmas which also could act as absorbing and scattering sites. A major disadvantage of such suspended particle type limiters is the reduction in transmissivity in particular spectral bands due to the suspending medium prior to breakdown. This may limit their use in these bands.
Devices for causing solid particles to oscillate back and forth between a pair of spaced electrodes in a fixed electric field are well known. Such a device first was employed by Andrew Gordon, a Benedictine monk, to produce a device known as the "electric bells" and was later used by Benjamin Franklin as the basis of the "Franklin Chimes" used to detect electrified clouds associated with thunderstorms. "B. Franklin Experiments and Observations on Electricity Made at Philadelphia in America by Mr. Benjamin Franklin communicated in Several Letters to Mr. P. Collison of London SRS London". Printed and sold by E. Caves at St. Johns Gate - 1751.
In an article by A. Y. H Cho entitled, "Contact Charging of Micron-Sized particles in Intense Electric Fields", Journal of Applied Physics, Vol. 35, No. 9, Sept. 1964, pages 2561-2564, oscillation of small solid particles in an electric field between a pair of spaced electrodes is shown.